1. Field of the Invention
The invention relates to a stack structure of a fuel cell battery.
2. Description of the Related Art
As disclosed in Japanese Patent Application Publication No. JP-A-7-249417, or as shown in FIG. 5, fuel cell batteries, for example, solid polymer electrolyte fuel cell batteries, have a stack structure that is formed by stacking unit cells 4. Each unit cell 4 is formed by sandwiching a membrane-electrode assembly (MEA) 2 between two separators 3, and the stack structure is secured with a fastening load applied in the cell stacking direction.
A central portion of each unit cell 4 in directions along the plane of the cell is an electricity generation region 5 that generates electricity upon supply of a fuel gas and an oxidizing gas to the MEA. A portion around the electricity generation region 5 is a non-electricity generation region 6 that seals in the fuel gas, the oxidizing gas, and cooling water. In the non-electricity generation region 6 of each cell 4, the two separators 3 form a fixed-dimension structure in which an electrolyte membrane 1 is sandwiched between hard resin frames 7 (the “fixed-dimension structure” herein refers to a structure in which one of the separators and the other separator or the separators and the electrolyte membrane bear loads by directly contacting each other without an intervening adhesive layer, and are provided with a uniquely defined dimension).
The fixed-dimension structure may include a pseudo fixed-dimension structure as well as the true fixed-dimension structure (the “pseudo fixed-dimension structure” herein refers to a structure in which one of the separators and the other separator or the separators and the electrolyte membrane bear loads by contacting each other through an intervening adhesive layer 8 that is thin (thinner than 50 μm) and is hard (the Young's modulus E thereof is greater than 100 MPa), and are provided with a uniquely defined dimension).
However, the conventional stack structure has the following problems.
1) The load on the MEA varies, so that the durability of the MEA reduces.
Since the constant thickness structure or pseudo constant thickness structure of the separators, and the MEAs have varying dimensions due to production and assembly, the load that acts on the MEAs upon application of a fastening load to the stack considerably varies and deviates from a target value. If the load on an MEA deviates from the target value to a larger side, the durability of the MEA reduces. If the load on an MEA deviates to a smaller side, the contact resistance of the MEA increases, and therefore degraded performance of the fuel cell results.
2) Large fastening load is needed.
The fastening load needs to be large so as to reliably attain a necessary surface pressure in the electricity generation region despite the variation of the load on the MEAs.
3) There is a possibility of cracking or deformation of separators due to the increased fastening load.
The application of extra fastening load raises the possibility of cranking of separators if they are made of carbon, and the possibility of deformation of separators if they are made of a metal.
4) Management of the MEA surface pressure is difficult.
Since the fastening load depends on the dimensional variations of the MEAs and the constant thickness structures or pseudo constant thickness structures of the separators, it is difficult to control the load on the MEAs by the stack fastening load.
The invention is intended to solve problems including 1) unstable durability of the MEA, 2) large stack fastening load, 3) possible cracking and deformation of separators, 4) difficult management of the MEA surface pressure, etc.